The invention relates to a method for machining flat workpieces in a double-sided machining tool, which has an upper and a lower work disk, wherein at least one of the work disks is rotatingly driven and the work disks each have an annular work surface, wherein the work surfaces amongst themselves limit an also annular work gap, in which at least one carrier is located, which guides at least one workpiece in the work gap, so that the at least one workpiece is machined in a double-sided manner between the work surfaces. For example, semiconductor wafers are machined, e.g. ground or polished in a double-sided manner with such machining tools. It is thereby decisive that the material removal is ended precisely at the desired point in time or respectively upon reaching a predetermined workpiece thickness. On one hand, a large number of workpieces should be produced with a smallest possible thickness deviation. On the other hand, the quality of the machined workpieces and the economic efficiency of the processes depend, in particular in the case of double-sided polishing, sensitively on the thickness of the machined workpiece in relation to the carrier receiving the workpiece. It can thereby be preferred to end the machining process if the workpiece is a few micrometers thicker than the carrier, for example a rotor disk. In this manner, abrasion and thus premature wear of the carrier are avoided. On the other hand, it can be preferred to machine the workpieces as precisely as possible to the same thickness as the carrier. This leads to an advantageous thickness distribution over the surface of the workpiece. It can finally be desired to further continue the machining process, for example a polishing process and to work, for example polish, slightly into the surface of the carrier. In this manner, an optimal local evenness, in particular in the edge area of the wafer, is achieved. The disadvantage of the two latter procedures is certainly an increased wear of the carrier, for example rotor disks, since they come in contact with the work coatings of the work surfaces. There is often only a difference in the thickness of the workpieces of 5-10 μm between the three possible named procedures. Against this background, it is absolutely necessary to end the machining of the workpieces precisely at the right time in order to reproducibly produce workpieces with the desired quality.
Solutions for measuring workpiece thicknesses are already known from the state of the art. U.S. Pat. No. 4,433,510 A thus describes the measurement of the distance between the work disks with an eddy current sensor. In order to avoid measurement errors caused by unevenness or incomplete parallel alignment of the work disks, the distance measurement thereby takes place only at certain times when the work disks are located in the same angle position with respect to each other. U.S. Pat. No. 5,969,521 A discloses an apparatus with at least two eddy current sensors, wherein it is ensured through the arrangement of the sensors with respect to each other that one of the sensors has a “free view” of the opposite work disk when the other sensor detects the presence of a rotor disk. Measurement errors caused by the rotor disks should be avoided in this manner. U.S. Pat. No. 7,147,541 A describes an apparatus for the thickness measurement of workpieces during double-sided machining with an eddy current sensor, which measures the distance of a work disk to the surface of a rotor disk. Work coating wear should thereby be compensated mathematically. Moreover, DE 10 2004 040 429 A1 describes a double-sided polishing machine that measures the distance between polishing disks at least two radially spaced points of the polishing gap. Based on this measurement, an undesired deformation of the work gap is detected and counteracted if necessary. An apparatus for influencing the work gap geometry, with which at least one of the work disks can be deformed concavely or convexly, is also known from DE 10 2006 037 490 A1.
The known methods for determining workpiece thickness often do not provide the required accuracy. In particular, measurement errors and thus inaccurate thickness determinations result from deviations in the gap geometry from the plane parallelism that are unavoidable or consciously brought about during operation.
Starting from the explained state of the art, the object of the invention is to provide a method of the initially named type with which workpieces can be machined more precisely up to a predetermined thickness.